I am finding out the phenomenon of the flow over the 2D cylinder using FLUENT.But I don't know how to calculate with unsteady flow,how to know the exact Time step size... and visual the vortex shedding in downstream flow over cylinder. in experience, with RE=150, they can see the alternate shedding, but i can't see. Help me, help me, help me right now.

Use small time step (begin with something like 1e-4), use k-e RNG, fine grid in the near (boundary layer) and after cylinder region (where the shed vortices will go). To visualize select contours of vorticity magnitude and set max limit (manualy) to something like 50. You have to make it clear if your case is Von Karman vortices or twin vortices (which do not shed).